Geologic Map of Three Sisters Volcanic Cluster, Cascade Range, Oregon By Wes Hildreth, Judy Fierstein, and Andrew T. Calvert Pamphlet to accompany Scientific Investigations Map 3186 Aerial view northward along glaciated summits of South Sister, Middle Sister, and North Sister volcanoes. Snow and ice- filled South Sister crater (rim at 10,358 ft) was created between 30 and 22 ka, during most recent of several explosive summit eruptions; thin oxidized agglutinate that mantles current crater rim protects 150-m-thick pyroclastic sequence that helped fill much larger crater. Middle Sister (10,047 ft) is capped by thick stack of radially dipping, dark-gray, thin mafic lava flows; asymmetrically glaciated, its nearly intact west flank contrasts sharply with its steep east face. Blue lake (near far right edge) is impounded by sharp-crested Neoglacial moraine. North Sister (10,085 ft) is glacially ravaged stratocone that consists of hundreds of thin rubbly lava flows and intercalated falls that dip radially and steeply; remnants of two thick lava flows cap summit. Broad mafic shield beyond North Sister is Black Crater; distant peak on horizon is Mount Jefferson; and Mount Hood is in dim distance. Photograph by John Scurlock, 2007. 2012 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank Contents Introduction.....................................................................................................................................................1 Physiography and Access ...................................................................................................................1 Previous Work .......................................................................................................................................1 Methods..................................................................................................................................................4 Geologic Setting .............................................................................................................................................4 Intra-Arc Extension...............................................................................................................................4 Cross-Oregon Crustal-Melting Anomaly ...........................................................................................5 Local Basement Rocks.........................................................................................................................7 The Quaternary Volcanoes ...........................................................................................................................9 The Mafic Periphery .............................................................................................................................9 Broken Top and the Tumalo Volcanic Field .....................................................................................10 North Sister ..........................................................................................................................................10 South Sister..........................................................................................................................................11 Middle Sister........................................................................................................................................11 Rhyolite Anomaly ................................................................................................................................12 Composition of Eruptive Products .............................................................................................................12 Postglacial Eruptions...................................................................................................................................13 Volcanic Hazards .........................................................................................................................................15 Acknowledgments .......................................................................................................................................18 Introduction to Description of Map Units ................................................................................................18 Description of Map Units ............................................................................................................................19 Surficial Deposits ................................................................................................................................19 Volcanic Rocks ....................................................................................................................................20 References Cited..........................................................................................................................................59 Figures 1. Regional location map of western Oregon, showing principal physiographic provinces, selected rivers and cities, and major Quaternary volcanic edifices of Cascade Range, including Three Sisters volcanic cluster. ..............................................................2 2. Map of Three Sisters region, showing locations of many geographic features mentioned in text. .......................................................................................................................................3 3. Tectonic setting of Quaternary Cascade arc. ............................................................................6 4. Rhyolite vents of Three Sisters and rear-arc Newberry caldera region. ..............................8 5. Chemical variation diagrams for more than 730 samples of Quaternary eruptive products of Three Sisters volcanic cluster. .......................................................................................14 6. Plots showing Zr, MgO, TiO2 Sr, FeO*, and Al2O3 versus SiO2 contents for eruptive products of Three Sisters volcanic cluster. .....................................................................16 Tables 1. Chemical data for Three Sisters volcanic cluster ....................................................................64 2. 40Ar/39Ar ages for Three Sisters volcanic cluster ....................................................................104 3. K-Ar ages for Three Sisters volcanic cluster ..........................................................................107 Sheets 1. Geologic Map 2. Correlation of Map Units and List of Map Units i This page intentionally left blank Introduction Highway (Hwy 46) to the south, and several Forest Service roads to the east. Most trails are accessible from July through The cluster of glaciated stratovolcanoes called the Three October, but for the rest of the year, owing to heavy snowfall, Sisters forms a spectacular 20-km-long reach along the crest few people (other than cross-country skiers) enter the wilder- of the Cascade Range in Oregon, 35 km west of Bend and 100 ness. Average annual precipitation on the crest of the Cascade km east of Eugene (fig. 1). As observed by trailblazing volca- Range, measured at Santiam Pass (elevation 4,817 ft [1,468 nologist, Howel Williams (1944), “For magnificence of glacial m], about 28 km north of North Sister), is 217 cm (85.3 in), scenery, for wealth of recent lavas, and for graphic examples and average snowfall is 1,145 cm (451 in) [Western Regional of dissected volcanoes, no part of this range surpasses the Climate Center (http://www.wrcc.dri.edu/index.html)]. Only area embracing the Sisters and McKenzie Pass.” The area we 8 percent of the average annual precipitation (and very little have now mapped in detail consists exclusively of Quaternary snow) falls there in the July-to-September quarter of the year. volcanic rocks and derivative surficial deposits. Although most Precipitation falls off sharply east of the crest, to annual aver- of the area has been modified by glaciation, the volcanoes are ages of about 34 cm at Sisters and 30 cm at Bend (fig. 1). Like young enough that the landforms remain largely constructional. the Sierra Nevada of California, the Three Sisters is a “gentle Locations of many of the geographic features mentioned in this wilderness:” its summer climate is usually moderate, and its report are shown in figures 1 and 2. relief, distances, and wildlife hazards are modest, yet it remains Scientific and journalistic interest in the Three Sisters spectacular and largely unspoiled. volcanic cluster was aroused a few years ago when ongoing The three principal volcanoes are similar in elevation: uplift centered about 5 km west of South Sister was identified, North Sister, 10,085 ft (3,074 m); Middle Sister, 10,047 ft first recognized by satellite imagery (interferometric synthetic (3,062 m); and South Sister, 10,358 ft (3,157 m). Their sur- aperture radar, InSAR) in 2001 (Wicks and others, 2002). rounding lava-flow aprons descend southward to about 5,440 Subsequent geodetic measurements and continuing InSAR ft (1,658 m) near Sparks Lake, northeastward to below 4,400 ft analysis confirmed 3 to 4 cm/yr uplift during the interval from (1,340 m) at Trout Creek Swamp and along Whychus Creek (on 1997 to 2004; the uplift has been modelled as inflation thought topographic maps, shown as “Squaw Creek”), and westward to to be caused by an intracrustal intrusion (Dzurisin and others, about 5,000 ft (1,525 m) near Indian Holes and as low as 3,500 2006, 2009), largely aseismic and plausibly involving mafic ft (1,065 m) at Linton Lake. To its southeast